The present invention relates generally to antiskid braking and traction control for a vehicle and, more particularly, to a system and method for providing such control in an electric or hybrid vehicle having a regenerative braking system operatively connected to an electric traction motor, and a separate hydraulic braking system.
Antiskid braking and traction control systems are well known and commonly employed in conventional internal combustion engine vehicles. However, the development of electric and hybrid vehicles has introduced concerns and opportunities unique to electric vehicle control system design. A primary area of concern in electric vehicle design, because of limited battery storage, is the amount of energy used by the various vehicle subsystems. Therefore, designing vehicle control systems that minimize energy use and conserve the available energy stored in the battery is critical. Vehicles driven by electric traction motors not only provide opportunities to conserve energy, but such vehicles also provide unique opportunities for energy regeneration. The braking and traction control systems of such vehicles are especially well-suited for such conservative and regenerative applications.
The kinetic energy that an electric or hybrid vehicle dissipates during braking, or any other period in which the accelerator pedal is not depressed and the vehicle is in motion, e.g. coasting, can advantageously be regenerated. Such regeneration can be accomplished by controlling the operation of the electric traction motor so that it behaves as a generator. The kinetic energy received during this process can be used to recharge the traction battery and stored for future use. Applying supplemental hydraulic braking only when the braking torque supplied by the electric traction motor cannot meet the driver's brake demand significantly increases the amount of energy recovered. The amount of kinetic energy that is wastefully dissipated while driving or launching an electric or hybrid vehicle is decreased if energy losses due to wheel slippage can advantageously be kept minimal. This can be accomplished by controlling the torque of the electric motor, independently of the driver's demand, through a conventional traction control slip regulator.
A regenerative braking system that maximizes the use of regenerative braking without causing locking or skidding of the vehicle wheels is shown in commonly assigned, U.S. Pat. No. 4,962,969 issued to Davis. However, the Davis patent does not teach or suggest minimizing the loss of kinetic energy, due to wheel slippage during acceleration, by controlling the torque produced by the electric traction motor during acceleration independently of the acceleration demand of the vehicle operator. In addition, the Davis patent does not provide for energy recovery by using regenerative braking during the antiskid braking mode of operation and does not take into account the cross-differential torque transfer through the differential or axle windup and damping which are significant during cornering and split mu road surfaces. Additionally, the Davis patent does not optimize energy recovery by only applying hydraulic brakes to the nondriven wheels with the exception of the driver demanding more braking than can be met with regenerative braking and maximum safe hydraulic nondriven wheel braking.
Accordingly, there is a need for a system and method for providing antiskid braking and traction control in an electric or hybrid vehicle which provides maximum regenerated kinetic energy during braking and minimizes the loss of kinetic energy due to wheel slip.